linux-kernel - [PATCH] btrfs: transaction: Fix misplaced barrier in btrfs_record_root_in

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Date:   Mon,  6 Sep 2021 11:25:59 +1000
From:   Baptiste Lepers <baptiste.lepers@...il.com>
To:     unlisted-recipients:; (no To-header on input)
Cc:     "Paul E . McKenney" <paulmck@...ux.ibm.com>,
        Baptiste Lepers <baptiste.lepers@...il.com>,
        Chris Mason <clm@...com>, Josef Bacik <josef@...icpanda.com>,
        David Sterba <dsterba@...e.com>, linux-btrfs@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: [PATCH] btrfs: transaction: Fix misplaced barrier in btrfs_record_root_in_trans

Per comment, record_root_in_trans orders the writes of the root->state
and root->last_trans:
      set_bit(BTRFS_ROOT_IN_TRANS_SETUP, &root->state);
      smp_wmb();
      root->last_trans = trans->transid;

But the barrier that enforces the order on the read side is misplaced:
     smp_rmb(); <-- misplaced
     if (root->last_trans == trans->transid &&
    <-- missing barrier here -->
            !test_bit(BTRFS_ROOT_IN_TRANS_SETUP, &root->state))

This patches fixes the ordering and wraps the racy accesses with
READ_ONCE and WRITE_ONCE calls to avoid load/store tearing.

Fixes: 7585717f304f5 ("Btrfs: fix relocation races")
Signed-off-by: Baptiste Lepers <baptiste.lepers@...il.com>
---
 fs/btrfs/transaction.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index 14b9fdc8aaa9..a609222e6704 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -437,7 +437,7 @@ static int record_root_in_trans(struct btrfs_trans_handle *trans,
 				   (unsigned long)root->root_key.objectid,
 				   BTRFS_ROOT_TRANS_TAG);
 		spin_unlock(&fs_info->fs_roots_radix_lock);
-		root->last_trans = trans->transid;
+		WRITE_ONCE(root->last_trans, trans->transid);
 
 		/* this is pretty tricky.  We don't want to
 		 * take the relocation lock in btrfs_record_root_in_trans
@@ -489,7 +489,7 @@ int btrfs_record_root_in_trans(struct btrfs_trans_handle *trans,
 			       struct btrfs_root *root)
 {
 	struct btrfs_fs_info *fs_info = root->fs_info;
-	int ret;
+	int ret, last_trans;
 
 	if (!test_bit(BTRFS_ROOT_SHAREABLE, &root->state))
 		return 0;
@@ -498,8 +498,9 @@ int btrfs_record_root_in_trans(struct btrfs_trans_handle *trans,
 	 * see record_root_in_trans for comments about IN_TRANS_SETUP usage
 	 * and barriers
 	 */
+	last_trans = READ_ONCE(root->last_trans);
 	smp_rmb();
-	if (root->last_trans == trans->transid &&
+	if (last_trans == trans->transid &&
 	    !test_bit(BTRFS_ROOT_IN_TRANS_SETUP, &root->state))
 		return 0;
 
-- 
2.17.1